1. Field of the Invention
The present invention relates to the field of marine survey data acquisition methods. More specifically, the invention relates to methods for acquiring high quality long-offset, full-azimuth survey data.
2. Description of the Related Art
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. WesternGeco L.L.C. currently conducts high-resolution Q-Marine™ surveys, in some instances covering many square kilometers. In many areas of the world hydrocarbon reservoirs located in structurally complex areas may not be adequately illuminated even with advanced towed marine streamer acquisition methods.
For example, the shallow, structurally complex St. Joseph reservoir off Malaysia produces oil and gas in an area that poses many surveying and imaging challenges. Strong currents, numerous obstructions and infrastructure, combined with difficult near-surface conditions, may hinder conventional survey attempts to image faults, reservoir sands, salt domes, and other geologic features.
A survey vessel known as a Q-Technology™ vessel may conduct seismic surveys towing multiple, 1000-10,0000-meter cables with a separation of 25-50 meters, using the WesternGeco proprietary calibrated Q-Marine™ source. “Q” is the WesternGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management. For additional information on Q-Marine™, a fully calibrated, point-receiver marine seismic acquisition and processing system, as well as Q-Land™ and Q-Seabed™, see http://www.westerngeco.com/q-technology.
To achieve high density surveys in regions having a combination of imaging and logistical challenges, a high trace density and closely spaced streamers may be used. However, this presents the potential of entangling and damaging streamer cables and associated equipment, unless streamer steering devices are closely monitored and controlled. Wide-azimuth towed streamer survey data is typically acquired using multiple vessels, for example: one streamer vessel and two source vessels; two streamer vessels and two source vessels; or one streamer vessel and three source vessels. Many possible marine seismic spreads comprising streamers, streamer vessels, and source vessels may be envisioned for obtaining wide- or rich-azimuth survey data.
Assignee's co-pending application Ser. No. 11/335,365, filed Jan. 19, 2006, discusses some of these. This document discusses shooting and acquiring marine seismic data during turns of linear marine surveys and during curvilinear paths. While an advance in the art, the art continues to seek improvements to marine seismic data acquisition techniques.
Cole, R. A. et al, “A circular seismic acquisition technique for marine three dimensional surveys”, Offshore Technology Conference, OTC 4864, May 6-9, 1985, Houston, Tex., described a concentric circle shooting scheme for obtaining three dimensional marine survey data around a sub-sea salt dome. While perhaps useful when the location of the feature is known, this technique would not be efficient or productive for finding new oil and gas deposits, or for monitoring changes in same if such information is desired.
A great leap in acquisition technology was described in another assignee's co-pending application, Ser. No. 12/121,324, filed on May 15, 2008. This reference describes methods for efficiently acquiring wide-azimuth towed streamer seismic data, which is also known as the “coil shooting” technique.
While the Q suite of advanced technologies for marine seismic data acquisition and processing may provide detailed images desired for many reservoir management decisions, including the ability to acquire wide- and/or rich azimuth data, the ability to acquire higher quality marine seismic data with less cost, or to increase the fold while also increasing the diversity of azimuth and offset, are constant goals of the marine seismic industry and would be viewed as advances in the art.